The present invention relates to digital wireless communication systems. More particularly, the present invention relates to a method of generating a list of neighboring active controllers and their identity information to facilitate handoff of a mobile communication.
In cellular mobile communication systems, a mobile handset communicates with base transceiver stations ("BTS's") positioned throughout a geographic area. Each BTS broadcasts on a discrete radio channel within a specified coverage area. A multiplicity of BTS sites may produce an array of cells to allow the system to provide radio coverage over a wide geographic area.
During a call, the user of a mobile handset will often move between neighboring cells within the system. As the mobile handset moves from one cell to a neighboring cell, the base station controller ("BSC") of the serving cell typically transfers the call session to the BTS of the neighboring cell. This transfer of the call is termed "handoff."
In digital cellular communication systems, such as those utilizing TDMA or CDMA signal transmission protocols, communications between multiple BTS sites and mobile handsets are carried out on radio frequency channels that can carry simultaneous communications. Using these digital protocols, communication sessions are typically performed using digitized voice or data signals that are transmitted as coded bursts or bursts within specified time slots. The coded bursts or time slots corresponding to multiple communication sessions are multiplexed onto the radio channels of the respective cells, and each mobile handset reads and communicates on an assigned decoded channels or assigned time slots on sending and receiving channels. Handoff in digital systems is typically performed using measurements from the mobile handset itself. Known as mobile-assisted handoff ("MAHO"), the method utilizes the mobile handset to periodically monitor and measure radio signals of neighboring BTS sites.
The list of neighbors that the mobile handset measures on a periodic basis is typically contained in a "neighbor cell list" of the cell that is presently serving the handset. The list may be kept on a database maintained at the site of the serving BTS, retained at a radio network controller ("RNC") site controlling a plurality of BTS's, or at a centralized mobile switching center ("MSC"). In order to compare measured signals from the neighbors on the neighbor list with the measured signal from the serving cell, the mobile station transmits the results of its measurements to the serving BTS. The serving BTS will in turn forward the measurements to either the applicable RNC's or to the MSC. If the received signal strength of the current channel in the serving cell falls below a threshold or is otherwise lower than the signal strength of a measured channel of a neighboring cell, the MSC or RNC may initiate handoff of the mobile handset's call session to the neighboring cell.
One advantage of digital CDMA or TDMA systems over analog systems is that mobile handsets have the ability to participate in "soft handoff." During soft handoff, the mobile handset communicates simultaneously with multiple base transceiver stations. The mobile handset establishes a new connection with a neighboring BTS before terminating the connection with the BTS of the serving cell. This allows the mobile handset to avoid call disruptions and interference during the handoff procedure.
In newer cellular infrastructure designs, it has become desirable to allow a single RNC or a plurality of RNC's to control more than one BTS. In a soft handoff situation, this configuration is advantageous because the controller for the serving cell may be able to communicate with a first controller for a neighboring cell to arrange handoff, process measurements or receive call data from the mobile handset, while the mobile handset may simultaneously communicate with a second controller for the neighboring cell to initiate the soft transfer of the communication. A multiple controller configuration would allow for smoother soft handoffs because the resources of one controller may be prioritized for measurement and transfer procedures and the other controller may focus on handling the actual multiplexed communication. In other situations, a cell having multiple controllers may be more receptive to transferred communications from neighboring cells because the additional number of controllers increases the cell's communication processing resources.
In present cellular systems, a BTS-handled call session is typically controlled by a single controller as specified by the present CDMA and TDMA standards. This controller is responsible for sending call information through the MSC to controllers or directly to controllers for neighboring cells during the measurement and handoff procedures. Each cell includes an associated neighbor list which includes information on which the BSC controls the neighboring cell. When the serving BSC determines the need for a hard or soft handoff to a neighboring cell which is not under its control, it must communicate with the controller of the neighboring cell. If more than one controller is operating on a neighboring cell, some procedure must be used to allow the serving controller to communicate with the active controller of the new cell.